1. Field of the Invention
This invention relates to a system for achieving a parallel relationship between a wafer surface and a reticle-pattern surface of a microlithographic stepper employing unit magnification optics.
2. Description of the Prior Art
In the fabrication of integrated circuits, a microlithographic stepper is used to image each of a stack of successive circuit-patterns on the surface of a silicon or GaAs wafer supported on a movable wafer stage of the stepper. The stepper, in response to control data applied thereto, is capable of precisely moving its wafer-supporting stage so as to bring each of the successively applied circuit-patterns of the stack into substantially perfect alignment with all the underlying circuit-patterns of the stack.
In particular, the wafer-supporting stage of a stepper is moved, with respect to a fixed base, to a particular X, Y coordinate position (in a substantially horizontal plane) in accordance with command position control signals received from a computer-controlled servo means. The computer-controlled servo means, like all servo means, is fed back information as to the then-existing actual position of the wafer-supporting stage, and then uses the error between this then-existing actual position and the command position to control the movement of the wafer-supporting stage in X and/or Y directions so as to reduce this error to substantially zero. However, the servo means, because it is computer-controlled, is capable of accomplishing this and other tasks in a sophisticated manner.
Specifically, the computer-controlled servo means normally includes an analog-to-digital (A/D) converter (unless the actual position information is already in digital form), a digital computer processing unit (CPU), memory means, and a stored program (which may comprise software) for controlling the operation of the CPU. Actual position information data from the wafer-supporting stage may be directly stored in memory, may be processed by the CPU before being stored in memory and/or may be employed, either before or after such processing, to modify the stored program. Further, the computer-controlled servo means may have manual data input means for selecting any one of different stored programs therein for the purpose of choosing any one of a plurality of separate modes of command-position control operation for the wafer-supporting stage. For example, in addition to including a stored program for implementing its normal operating mode for controlling the movement for the wafer-supporting stage, the computer-controlled servo means also may include a stored program for implementing a certain calibration-mode operation of movement for the wafer-supporting stage. Alternatively, the stored program itself may automatically choose such a calibration-mode operation at certain specified programmed times. In general, the computer control of the servo means permits any desired mode of operation thereof to be realized in determining the command position control signals applied to the wafer-supporting stage and/or in processing the actual position information data received from the wafer-supporting stage.
The wafer stepper includes suitable mechanical means including motor means coupled to the wafer-supporting stage capable of providing the stage with translational motion relative to a fixed base of the wafer stepper with respect to each of the three mutually orthogonal axes X, Y and Z (where the axes X and Y are substantially horizontally oriented and the axis Z is substantially vertically oriented). Further, this motor means is capable of providing the wafer-supporting stage with rotational motion about each of axes X, Y and Z. By coupling the aforesaid computer-controlled servo means to the mechanical means of the wafer stepper, precise control of both translation and rotation of the wafer-supporting stage is achieved.
While it is possible to employ contact printing for imaging a circuit-pattern on the surface of the wafer, it is more practical to employ a projection optical system for this purpose. One type of such projection optical system, now known as a Half-Field Dyson projection optical system, is disclosed in U.S. Pat. No. 4,964,705, entitled "Unit Magnification Optical System," which issued Oct. 23, 1990 to David A. Markle, and in its continuation-in-part U.S. Pat. No. 5,040,882, entitled "Unit Magnification Optical System with Improved Reflective Reticle," which issued Aug. 20, 1991 to David A. Markle (both of the aforesaid patents being assigned to the same assignee as the present patent application). The teachings of both of these patents are incorporated herein by reference. Specifically, an advantage of Half-Field Dyson projection optical system is that it is particularly suitable for projecting an image of a reflective reticle integrated-circuit layer pattern, that occupies a relatively large optical field, on the surface of a wafer.
The features of both a reticle integrated-circuit layer pattern and its image on a wafer surface have microscopic dimensions. This means that a high numerical aperture is required of any projection optical system, such as the Half-Field Dyson projection optical system, in order to obtain a high-resolution image of the integrated-circuit layer pattern on the wafer surface. Such a high numerical aperture projection optical system has a microscopic depth-of-focus. Further, the thickness of a wafer varies from one wafer to another. The problem is then to make sure that the surface of a wafer is controllably moved by the stepper to that position where the wafer's surface substantially coincides with the image plane of the projection optical system of the stepper.
Reference is made to the teachings disclosed in the aforesaid cross-referenced copending patent application Ser. No. 993,547. This patent application discloses a technique which makes it possible to precisely position the wafer's surface with respect to the reticle by employing an image of a repetitive diffraction pattern on the reticle which is focused only on a small spot on the wafer's surface (i.e., the small spot itself substantially coincides with the image plane of the projection optical system of the stepper). However, the entire area of the wafer's surface will not substantially coincide with the image plane of the projection optical system of the stepper unless the wafer is also controllably adjusted by the stepper to that position where the wafer's surface is angularly oriented substantially parallel to the reticle's surface. The present invention is directed to apparatus for accomplishing this.